Background. The spotcheek emperor, Lethrinus rubrioperculatus Sato 1978, is an important component of the mesophotoic coral reef community in the Indo-Pacific Region, where it is a target of commercial fisheries. A diet composition study was undertaken as part of a broader investigation towards elucidation of the biological characteristics of this species in the Commonwealth of the Northern Mariana Islands, Micronesia. Materials and methods. A total of 395 spotcheek emperor stomachs were examined between September 2000 and February 2002 based on a commercial fishery and research cruises.  Feeding activity was estimated using indices of vacuity and repletion, diet composition was defined using the Main Food Item (MFI) index, and trophic level was determined using Trophlab. All indices were calculated to observe for patterns demographically, seasonally, ontogenetically and by sex. Results. Total vacuity index (VI) for fish was 45.8% while inclusion of bait in stomach (BIS) raised VI to 60.5%. The highest VI came from Farallon de Medinilla, with 63% and 73% including BIS. Lowest VI was at Marpi Reef, while Sonome Reef had the largest separation between VI and VI BIS. Repletion indices were greatest in Saipan and Tinian, lowest at Sonome Reef. First quarter year had the highest vacuity and lowest repletion. Repletion increased from the first to fourth quarter while VI did not reveal a pattern. Repletion decreased with size, and VI was highest in the smallest size group. Repletion and VI values were greater in females. MFI results showed fish were the primary prey item at all locations, seasons, and sizes, and in both sexes. Fish increased in diet seasonally. The smallest size group had the highest fish MFI, and the largest size group the lowest. Crustaceans and mollusks were secondary prey items varying by location, season and size. Saipan and Tinian uniquely displayed five prey categories. Overall trophic values were estimated at 4.23 ± 0.73, with a range from 3.96 to 4.46 as estimated by location, season size and sex. Conclusion. Results differed from New Caledonia that reported fish as a secondary food item to crustaceans for spotcheek emperor. In this study mollusks then crustaceans were present as accessory prey items. Demography, ontogeny, and size influenced diet. Estimated trophic levels were indicative of a piscivorous diet, in contrast to the value provided by FishBase. Future work in elucidating observed trends in diet preference for the spotcheek emperor should focus on obtaining a much larger sample size that is more evenly distributed across location, season, size class and sex.

Lethrinus rubrioperculatus, diet composition, Northern Mariana Islands, trophic level